SOUTH SAN FRANCISCO — Derrick Gaines would have turned 16 Thursday, but instead of throwing him a birthday party, his family and friends mourned him at the gas station where police shot him dead during a June confrontation.

At the Arco Station on Westborough Boulevard, nearly 100 people, many of them black or Latino teens, remembered Gaines as his mother, Rachel Guido Red, acknowledged her son made some youth-related mistakes.

Even so, she went on to say that he will never have a chance to mature into a man as a result of that fatal encounter.

“He could have done a whole lot of things besides kill this kid, rob him of his life,” Guido Red said of South San Francisco police Officer Joshua Cabillo. “There will never be another Derrick.”

The three binder-thick report backing the conclusion contains harrowing details of the June 5 shooting that occurred as Cabillo and Gaines were just inches away from one another. It provides clues to where Gaines may have gotten his broken Colt .45 revolver that Cabillo said the teen was reaching for when he fired.

Gaines was shot once in the neck and died at 9:46 p.m. that night.

“I wanted to save the kid’s life. I wanted to tell him something — but the words didn’t come out,” Cabillo said in an interview with investigators. “At this point I can see it’s a young child.”

Cabillo broke down in the interview, according to the police report.

An hour earlier, Gaines’ mother walked into the South San Francisco police station, demanding to know if police had shot her son. After getting the news she dissolved into grief.

Cabillo had been near the Arco station at the intersection of Westborough and Gellert boulevards about 9 p.m. that night to investigate possible illegal gambling in the area, the report says.

Cabillo, a cop since March 2008 and member of the street crimes team, was stopped at a traffic light on Gellert Boulevard when he spotted Gaines and Remy Carrillo, then 14, crossing the street. Carrillo told investigators the boys were walking home after the 122 Stonestown bus on Gellert Boulevard didn’t show up. The last northbound bus would have passed the stop about an hour before the shooting.

Police say the boys abruptly halted, made a 180 degree turn and started walking away as soon as they laid eyes on Cabillo’s cruiser.

While the officer watched the boys, he saw Gaines fidgeting with something in his waistband. Cabillo later told investigators it could only be one of two things: a gun or an effort to hide drugs. He decided to follow the teens. As he did so they kept peering over their shoulders at him, the report says.

After his death police found a small quantity of methamphetamine and marijuana on Gaines. Investigators also pulled text messages from the boy’s cell phone detailing drug transactions and attempts to get another gun. Several friends told investigators he carried the pistol, even though it was unloaded and had no firing pin, for protection from “drug fiends.”

Gaines’ mother said the information is irrelevant to her son’s death.

“They always want to taint the victim’s persona,” said Guido Red. “Is that a justifiable reason to kill him?”

Cabillo got out of his cruiser and walked up to the two at the Arco station. Gaines took off running, and as he did the officer says he saw the butt of the pistol poke out from the boy’s clothing.

Cabillo ran Gaines down and knocked him to the ground, where the teen ended up on his back, the gun laying a foot from his left knee, the report says.

By that time, Cabillo was aiming his department-issued handgun at Gaines at point-blank range. The teen reached his right arm across his body, toward the pistol, Cabillo told investigators. As Gaines moved, his neck butted up against the muzzle of the officer’s gun.

“I remember in my mind, I couldn’t believe he was doing that,” the officer told investigators. “I was in shock that he was actually going to reach for the gun.”

Cabillo, fearing for his life, shot once and tried to fire again, but his gun jammed. The officer said he was, “praying for the kid,” as he started CPR.

The only thing police know for sure about Gaines’ weapon is that Colt shipped it to Simms Hardware Co. at 2801 J St. in Sacramento on May 10, 1962, just about one month shy of 50 years before Gaines death. At the time there was no requirement for gun sellers to record who bought a gun, so after that the trail goes cold.

A day after Gaines’ death, a man turned up at San Francisco General Hospital severely beaten. Police learned he may have supplied the Colt to Gaines, but the man told investigators a family dispute had sparked the violence.

A police search of Gaines’ home turned up assorted ammunition in the family garage, including Colt .45 shells. The boy’s uncle had owned guns, but the family said they had been destroyed after he died.

Tragically, a couple days before the shooting, Gaines’ mother would tell police, she had heard metallic clicking noises coming from her son’s room. She thought they could be made by a gun, but she didn’t confront him. It was late and she didn’t want to start a fight, police said.

“I was going to search his room that night and I didn’t,” said Guido Red. “You always ask, ‘What if?’ I should have, would have, could have.”